The present invention relates to liquid cooled assembled pistons for internal combustion engines.
To comply with steadily increasing requirements of the market, a designer of pistons for modern high duty diesel engines for medium speeds must furnish the engine manufacturer with pistons which are abreast of the technical development of the engines. A steady increase in functional reliability and service life are of special importance. High combustion pressures and combustion temperatures must be achieved with practicable technical means, which depend on the material and the processing thereof and also on economic considerations. In cooperation with the engine manufacturer, the piston designer usually finds a compromise, which represents an optimum with respect to costs and performance. At the present state of the art, all requirements regarding factor of safety and service life are met by a cooled, assembled piston.
To minimize the dimensions and weights of high-duty diesel engines, it is known to make the upper part of the piston from a heat-resisting ferrous material, particularly low-carbon steel, to make the lower part from an aluminum alloy for pistons, and to connect the two parts to each other by tie rods or screws. Adjacent to the interfacial plane between the parts of the piston, cooling chambers are provided for dissipating the heat which is contained in the upper portion of the piston inasmuch as the heat cannot be dissipated otherwise.
Such pistons usually have a relatively shallow combustion chamber recess so that the highest piston head temperature, lying above 350.degree. to 400.degree. C., occurs at the oblique outer edge of the recess owing to the configuration of the jets of fuel injected through nozzles. Temperatures of about 240.degree. to 270.degree. C. may be obtained at the corresponding portion of the inside surface of the cooling passage, which surface is wetted by coolant oil. These temperatures result in yellow to blue temper colors on the steel surface and are close to or above the spontaneous ignition temperature of commercially available lubricating oils for diesel engines.
The experience had with such pistons in practice has confirmed the belief that the coolant oil cokes very rapidly in that portion of the cooling passage and forms an insulating oil coke layer which reduces the cooling action so that the temperatures are greatly increased, the strength properties of the piston material are decreased, the creep strength is reduced and the thermal deformation is increased. It has repeatedly been observed that this may result in permanent deformation.